Over the last two days we made a big step forward in preparing the Rappahannock sperm whale. We were finally able to remove one of the dentary fragments that was stuck to the skull. The photo above shows the the skull jacket on May 12. Of the two fragments labeled “dentaries”, we removed the one on the right (which turns out to be the right dentary). Below is the same bone after removal and reconstruction:
When this whale died, the lower jaw broke loose from the skull and rotated 90 degrees, and then the skull landed in the mud on top of the back ends of the jaws, crushing them. When the piece was removed it had to be reconstructed in three dimensions.
Each dentary has an opening that runs along its length called the mandibular canal, which carries nerves and blood vessels in the living animal. Near the back end of a whale’s dentary the mandibular canal becomes very large, and eventually the bone that forms the inside (medial) edge of the canal ends. This leaves a plate of bone (sometimes called the pan bone) on the outer edge of the mandibular canal. The pan bone continues back to the condyle, the part of the dentary that articulates with the skull. In modern toothed whales the pan bone is part of the structure for transmitting sound to the ears.
If we flip the sperm whale’s dentary over we can see the medial side:
The fragment preserves the inside back edge of the mandibular canal (or, to be technical, the posteromedial margin). Only the front part of the pan bone is preserved; the back half and the condyle are missing. Hopefully the front part of the dentary (which is still in its jacket) will attach to the broken edge on the left side of this bone.